The present invention is directed to a variable microwave signal attenuator, and more particularly to such an attenuator utilizing PIN diodes as variable resistance elements.
Various attempts have been made to devise an attenuator circuit that will exhibit an impedance which is variable but is substantially independent of frequency over a wide range of microwave frequencies. Additional desirable characteristics are a minimum change in phase shift with changing attenuation, and high switching speeds so that the degree of attenuation can be quickly varied. In such devices, it is common to employ PIN diodes coupled in some fashion to a transmission line to act as variable impedance elements. In most applications, e.g. radar or communications systems, it is desirable that the impedance of any device coupled to the transmission line match the characteristic impedance of the line, since an impedance mismatch will cause a portion of the RF signal to be reflected at the mismatch junction.
A common technique is to couple PIN diodes in shunt between the transmission line conductor and ground as disclosed in U.S. Pat. No. 3,775,708 to Sly, or to arrange the PIN diodes in a .pi.-pad configuration as disclosed in U.S. Pat. No. 4,010,430 to Wolfe. In such circuits, a D.C. bias current varies the effective resistances of the series and shunt diodes in order to control the degree of attenuation, but these circuits typically suffer from very high phase shift at maximum attenuation. In an attempt to reduce the phase shift, a "double-.pi." arrangement has been proposed by Williams in U.S. Pat. No. 4,097,827, with resistors being connected in shunt with the series diodes to thereby minimize the phase-shift effect of the diode junction capacitance. As a further solution to the phase-shift problem, U.S. Pat. No. 4,009,456 to Hopfer describes a Tee-Pi configuration using four PIN diodes with two of the diodes being coupled in series with a transmission line and two being coupled as parallel shunt resistances between ground and the common connection point of the two series-connected diodes. In the Hopfer configuration, phase shifts are minimized by reducing the PIN diode parasitics as much as possible.
Although these various PIN diode attenuator circuits are acceptable in a number of applications, they all still suffer from excessive phase shift at increasing degrees of attenuation. Further, some configurations utilize a plurality of series-connected diodes and, when changing the degree of attenuation, one of the diodes will inevitably turn off first and the remaining diodes will then have to discharge through the high impedance of the "off" diode. This substantially limits the switching speed of the attenuator.
Still further, although some circuits have succeeded in improving the phase shift characteristics, they have invariably done so at the expense of increased circuit complexity and consequent higher fabrication costs.